The protein secreted by HeLa cells that cross-reacts with antiserum developed against the alpha-subunit of human chorionic gonadotropin (hCG) has been purified approximately 30,000-fold from concentrated culture medium. It has a specific activity (by RIA) of 6.8 x 10[unreadable]5[unreadable] ng alpha/mg protein and appears homogeneous by electrophoresis on reducing/denaturing polyacrylamide gels. Amino acid analysis indicates that Hela-alpha has a composition very similar to that of the urinary hCG alpha-subunit, with the possible exception of Leu and Lys, which are higher and lower, respectively. Peptide fingerprints of the HeLa protein and hCG-alpha indicate that several of the Tyr-, Met-, and Cys-containing tryptic peptides are held in common, but other comparisons have demonstrated that the tumor protein is not identical to its normal counterpart. HeLa-alpha elutes prior to CG-alpha during Sephadex G-75 chromatography, suggesting a higher apparent M[unreadable]r[unreadable], but migrates faster during SDS-PAGE, suggesting a lower M[unreadable]r[unreadable]. This discrepancy can probably be accounted for by differences in their carbohydrate constitutents. Isoelectric focusing of the HeLa and CG subunits demonstrates that the tumor protein has a lower pI, and removal of NeuNAc by mild acid hydrolysis does not entirely eliminate this difference. Immunopreciptiation and electrophoresis of alpha from HeLa cultures labeled with [[unreadable]3[unreadable]H]fucose indicate that the tumor subunit is fucosylated, whereas analysis of CG-alpha hydrolysates by HPLC confirm previous reports that the placental subunit does not contain fucose. HeLa alpha-subunit is unable to combine with hCG beta-subunit to form holo hCG under conditions where the hCG alpha-subunit is able to do so. The differences observed may be useful in diagnosing neoplastic vs. hyperplastic conditions and may lend insight into the mechanism of ectopic hormone production by tumors. (C)